scsi: documentation: add scsi_common.c to SCSI driver-api

Add exported functions from scsi_common.c to the SCSI driver API
documentation.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>